Visibility meter



April 25, 1950 1 a E. o. HULBURT 2,504,981

VISIBILITY METER Filed Aug. 8, 1946 DETECTOR PROJECTOR COUNTING CIRCUIT INDICATOR I .I |E'r- L EDWARD O. HULBURT Snow-mg Patented Apr. 25, 1950 VISIBILITY METER Edward 0. Hulburt, Alexandria, Va.

Application August 8, 1946, Serial No. 689,134

Claims.

(Granted under the act of March 3, 1883, as amended April 30,. 1928; 370 0. G. 757) This invention relates to visual range meters,

and has particular reference to a device for the determination of visual range through use of a projector of invisible electromagnetic radiations combined with a detector therefor.

An object of the present invention is to provide a device capable of shipborne or airborne use which will objectively determine visual range at all times of day and under all weather conditions.

Another object of the invention is to provide an apparatus which will determine visual range by the use of invisible radiations, thus" enabling it tobe operated at night with a maximum of security aboard military ships and aircraft.

Another object of the present invention is to provide an apparatus which may be used at airports'and aircraft landing areas'to determine in an accurate manner the ceiling or upper limitof visibility.-

f Other objects and advantages of our invention will in part-be obvious and in part appear hereinafter.

In the navigation of ships and aircraft, as well as in the early detection of other ships and aircraft it is important to ascertain the range'of optical visibility under the'varying weather conditions which may prevail. While such visibility may be estimated by optical means alone, such methods become unreliable at night and under. conditions in which no comparison standard is available, such as another ship or aircraft of approximately known distance. At such times it is particularly lmportantthat some objective means be provided, in order that a zone of safety-be determined and anavigable speed be set.

I A device for the objective determination, ofv

visibility may be made to comprise a radiation source, and means for measuring theradiation scattered by the dust and moisture particles in the atmosphere. While such radiation may consist of visible light it is preferable to use somewhat shorter wavelengths of the order of 2900 Angstroms or less, since less interference will be encountered from the sunlight at these wavelengths. 'Ultraviolet radiation possesses very f 2900 Angstroms wavelengths or less which may be provided with a filter to limit the type of radiation, in combination with a Geiger-Mueller counter tube sensitive to these radiations and so placed in relation to the projector that it receives only the radiations reflected from dust and haze particles in the atmosphere. Coupled with the Geiger-Mueller counter tube itself is a counting circuit and indicator designed to receive the pulses produced by the counter tube, to count them, and to indicate the average number of pulses per second, which indication may be calibrated to give a direct reading of the visual range.

The fact that the counter tube produces pulses is due in no wise to a pulsation in the reflected radiation, since the latter is steady and nonintermittent. The inherent characteristics of the Geiger-Mueller counter tube are such that between the time that a discharge is started by an ionization of the gas within the tube and the time that the internal or external quenching means have operated to prepare the tube for its next discharge, a dead time occurs in which the tube does not indicate the presence of an ionizing medium.

For a better understanding of the present invention, reference may be had to the accompanying drawings wherein:

Figure 1 is a block diagram showing the interrelationship of component parts of the apparatus-described in this invention.

Figure 2 is a schematic diagram of a preferred form of Geiger-Mueller counter tube for use as the ultraviolet sensitive detecting element.

Figure 3 is a diagram showing the arrangement of an embodiment of the projector-detector unit for this invention.

Figure 4 is a diagram showing a structural modification of the projector-detector unit.

The basic arrangement of parts, as shown in Figure 1 comprises a projector which is so designed that it transmits a beam of ultraviolet radiation of wavelengths of 2900 Angstroms or less in a controllable direction. A primary source of such radiation may be an incandescent lamp, an arc lamp, or a radiation emitter of the vapor excitation type such as the mercury vapor lamp. The source should preferably be rich in ultraviolet radiation and of an adequate output to provide a proper amount of reflected radiation to be detected by the Geiger-Mueller tube. Since many radiation sources produce considerable visible light it is necessary, in most cases, to use an ultraviolet filter in conjunction with the radia-.

tion source so as to eliminate the visible components and to narrow the radiation spectrum in order to prevent interference from the sun and other sources of extraneous electromagnetic waves which may act upon the detecting tube.

The detecting element comprises a Geiger- Mueller counter tube, embodiment of which is shown in Figure 2, The tube comprises a glass envelope, 20, within which is a wire anode, 2|, surrounded by a cylindrical cathode, 22 made of a metal such as an alkali metal, which is par-' ticularly sensitive to ultraviolet radiations. The window, 23, is of some material such asguartz which is highly permeable to these radiations. Electrical connections are made through leads 24 and 25. The interior of the counter tube contains an inert gas, such as krypton, andffor fast counting action, some organic yaper sueh ae methylene bromide or ethyl alcohol which is capable of quenching or deionizing the tube in a er ho t ime ie lewihe the itia io z a tion Afiilter may be combined with the detector we tha h la ter re hehde on to h er ee resiiaiiene mitte h t e eie e r use of such a filter the tube is rendered sens ti e t th r d e ieh a are being jected, as a consequence of which interference d e 9 eQl r .i fil ex ra s Sources i e heed to a minimum.

Cou led with the Q iee Mi el counter tube e n eleet ehie .e ihe. eireu t r c n an ut ut elteee-ereeeriieeel to the. n m rs of Pulse h re ue d hr theeehn er T a t which pulses are produced is proportioned to the intens ty o t e stee rredia ieh eehe he io i eh with n the t e, and t e puls n s u o uee i e h ti nd-qu nch ng of th u he ind. a .a ircu t. n edue a rea in there ere W... is ropo ti nal .129 the eye aee at at wh h the ube-i eouht he- Since. in he on etieh eithis nrera ue di relati shi exists. bet een thente sit o efle ed r iaen from du ancii e n e-tides. an th o tns ra e. 9f the G i er-Mue r tubeth i dicator may e eel hra d .to,. :eal.e re.et in ard o miles of optical visibility.

The. neeifie mbodiment re 2 5 c mponent earner thi ih eht d pen u e theparticuiar application for w ch-itais. tobe adapted,

Fiehre .s he.whe. ie r eheemeh o a 1.

ined. ie ie dete te unit. wh h the. Gei e -M l cunter tub is ri dly atta hed t9. the reie er unit ahdso orient d that its axis at en tiv s etel timerl re tep.ara11e1 to. the am from therroiecten The. comb n unit. support d-b irame .0, c m r se a project or housing 3! containing a source of radiation 32 and reflecting means 3-3, Filter 34 is eetee i t e. ro ected bea n- The ieer- 1- ler countertube, 35 is mounted in a shielded housing, 35, which is aiiigged to the projector by races; ereefih r firmer be a e the ube e e teim he radi ti ns ea g the detector. w

F r ehe e. medifieetieh. Qf't ie; r an ement in which the detectoris mounted within th p eie e he ne a d ma s the sam fi e he Wiener-d tor un 4 is upq f eeby r e. it-end. empri e a u e of radiati iz andrefiecting surface 43. Mounted I el y th, th ezs ee the transmitt d beam isa housing :44 con aining the Geiger- Mueller counting tube 4;. The filter may be ue fier. beihte. i ahdde e ti l me ts.

I 11 t e.- erehen. e nrehtien- 11 331 115.? be.

realized that while a direct correlation exists between the refiectivity of the atmosphere and the number of counts on the indicator there must be an empirical calibration of each particular 5 modification of the apparatus. It will further be apparent that the uncorrected readings obtainable at daytirneand at nightwill bedifierent because of the ef fectof so jr radi-ation. {Therefore a'j'zero setting of the indicator must be made with l0 the projector off, and when the radiation source 'is turned on, the reading obtained may be aslsumed-to be a direct result of the radiation reflected fronrthe atmosphere.

The device may be installed aboard ships where ,ititvould behighlyuseful for determining visibility .corldi iiQns, particularly at night. It has widespr'ead'application as an airborne device for navicgailionaluse and asameans, when carried aboard search aircraft, :fordetermining the visible range within which an object may be sighted. The ap- Daraths described he ein iurther-ibehsede a im ans'f r d termin ng th ceiling or u p vi ih ityslr at a 1 di g field. which inf mes reau rediat i. seem interva s in eva u A. methedeideterm ni g risihl'e ranee in. h t n sphere. eemn sin projec in -nonunerm t pt-e ectroma netic'radia ien of less tha about 290.Q=.Angs,tr0ms wavelen th intothe atmosphere, detecting; radiation. reflected .from suspended solid; and liquid particles :inthe..-.air with. a Geiger-Mueller v.cournier, and converting the: signal. from saiddetectiominto a sensible: indica-.

ti-onof the intensity'orsaid reflected: radiation in terms of visible range.

:3. Amethod of determining -visibl'e. range in: the atmosphere comprising, projecting filtered non-intermittentelectromagnetic radiation of less than about. 2900 Angstroms wavelength into. the atmosphere, detecting radiationrefiected f-rom; suspended solid and liquid particles. in the airwith a Geiger?lmiellenv counter after-re-filtering,

and: converting t'hesignal from said detectioninto REFERENCES CITED The following references are of record in the file of this patent:

6 UNITED STATES PATENTS Number Name Date 1,385,657 Bell et a1 July 26, 1921 2,206,036 Herson July 2, 1940 2,237,193 Mobsby Apr. 1, 1941 FOREIGN PATENTS Number Country Date 522,890 Great Britain July 1, 1940 OTHER REFERENCES Applications of Germicidal, Erythemal and Infrared Energy, Luckiesh, D. Van Nostrand (30., 250 Fourth Ave., New York City, page 20.

Ser. No. 326,650 (A. P. 0.), published May 18, 1943. 

